CWL: general config support; add pointer to database
CWL: general config support; add pointer to database

Enables usage of pre-created MySQL databases by passing in a
custom configuration. General to allow overrides of other part of the
bcbio Cromwell configuration preparation in the future.

lbeltrame/bcbio-nextgen,lbeltrame/bcbio-nextgen,a113n/bcbio-nextgen,lbeltrame/bcbio-nextgen,a113n/bcbio-nextgen,chapmanb/bcbio-nextgen,vladsaveliev/bcbio-nextgen,chapmanb/bcbio-nextgen,chapmanb/bcbio-nextgen,vladsaveliev/bcbio-nextgen,vladsaveliev/bcbio-nextgen,a113n/bcbio-nextgen